Cosmic microwave background radiation
Cosmic background microwave radiation (CMB radiation) is radiation in the microwave part of the electromagnetic spectrum, which comes from all directions in outer space. We know that it comes from very far away, so we think that it is the oldest signal that we can detect.
According to the Big Bang theory, when the universe began, a lot of high-energy radiation was created. Then, the universe became bigger and colder. Therefore, the high-energy radiation lost a lot of its original energy. Now, as a result, that radiation is in the microwave part of the electromagnetic spectrum (the microwave part has quite low energy).
Arno Penzias and Robert Wilson first detected the CMB radiation. Scientists think that the existence of CMB radiation is very important evidence along with red shift theory that the Big Bang theory is true.